Influence of the anion of the salt used on the coordination mode of an N-heterocyclic carbene ligand to osmium

The reactions of the hexahydride OsH6((PPr3)-Pr-i)(2) (1) with 1.0 equiv of the BPh4-, BF4-, and Br- salts of 1-(2-pyridylmethyl)-3-methylimidazolium, in tetrahydrofuran under reflux, have been studied. In the three cases, mixtures of the abnormal [OsH(eta(2)-H-2){kappa C-5,N-[1-(2-pyridylmethyl)-3-methylimidazol-5-ylidene]}((PPr3)-Pr-i)(2)]A (A = BPh4 (2a), BF4 (2b), Br (2c)) and normal [OsH(eta(2)-H-2){kappa C-2,N-[1-(2-pyridylmethyl)-3-methylimidazol-2-ylidene]}((PPr3)-Pr-i)(2)]A (A = BPh4 (3a), BF4 (3b), Br (3c)) isomers are obtained. The formation rate of the abnormal isomer and the abnormal to normal ratio decrease as the coordinating power of the anion of the used salt increases. Treatment of 2b with either HBF4 center dot OEt2 or LiBF4 gives rise to its isomerization to 3b. The X-ray structure of 2a and T-1(min) values of the OsH3 resonances of the cations support the hydride-elongated dihydrogen nature of these compounds. The nonclassical interaction between the hydrogen atoms of the OsH3 unit is more important in the normal isomer than in the abnormal. Treatment of 1 with 2.0 equiv of 1-(2-pyridylmethyl)-3-methylimidazolium bromide yields the bis(normal-NHC) complex [OsH{kappa C-2,N-[1-(2-pyridylmethyl)-3-methylimidazol-2-ylidene]}(2)((PPr3)-Pr-i)]Br (4), which has been also characterized by X-ray diffraction analysis.